Simple, rude, rate limiter (using memcache).

ratelimiter.php

<?php
/* Very simple, crude, rate limiter */

/*
Assumes _MEMCACHEDSITEPREFIX, _MEMCACHEDHOST and _MEMCACHEDPORT are `defined`:

define('_MEMCACHEDHOST', '127.0.0.1');
define('_MEMCACHEDPORT', 11211);
define('_MEMCACHEDSITEPREFIX', 'mysite');
*/
class RateLimiter {
    private $rate_limit_cycle;  //Seconds
    private $cycle_req_quota;   //Reqs/cycle
    
    function RateLimiter($rate_limit_cycle = 86400, $cycle_req_quota = 5000) {
        $this->rate_limit_cycle = $rate_limit_cycle;
        $this->cycle_req_quota = $cycle_req_quota;
    }
    
    public function IsLimited($key, $prolong_on_rate_limit_exceeded = false) {
        $memcache_id = _MEMCACHEDSITEPREFIX . '.ratelimit.' . $key;

        $memcache = new Memcache();
        $memcache->connect(_MEMCACHEDHOST, _MEMCACHEDPORT);
        
        $current_hits = intval($memcache->get($memcache_id));
        
        if ($current_hits >= $this->cycle_req_quota) {
            if ($prolong_on_rate_limit_exceeded) {
                //When rate limit is hit we prolong the key to the rate_limit_cycle number of seconds ("extra punishment")
                $memcache->set($memcache_id, $this->cycle_req_quota, 0, $this->rate_limit_cycle);
            }
            return true;
        }
        
        if ($current_hits == 0)
            $memcache->set($memcache_id, 1, 0, $this->rate_limit_cycle);
        else
            $memcache->increment($memcache_id);
        return false;
    }
    
    //Returns if the request is rate limited; returning an error etc. is up to you
    public static function CheckLimit($key, $rate_limit_cycle = 86400, $cycle_req_quota = 5000, $prolong_on_rate_limit_exceeded = false) {
        $r = new RateLimiter($rate_limit_cycle, $cycle_req_quota, $prolong_on_rate_limit_exceeded);
        return $r->IsLimited($key);
    }
    
    //Checks the rate limit for a specific key and exits with some HTTP headers when the rate limit is hit
    public static function CheckLimitExit($key, $rate_limit_cycle = 86400, $cycle_req_quota = 5000, $prolong_on_rate_limit_exceeded = false) {
        if (RateLimiter::CheckLimit($key, $rate_limit_cycle, $cycle_req_quota, $prolong_on_rate_limit_exceeded)) {
            header('HTTP/1.1 429 Too Many Requests');
            header(sprintf('Retry-After: %d', $rate_limit_cycle));
            header('Connection: close');
            echo '<h1>Too many requests</h1>';
            exit();
        }
    }
    
    //RateLimiter::CheckLimitByIPExit();                        //5000 requests per day
    //RateLimiter::CheckLimitByIPExit(null, 10, 5);             //5 requests per 10 seconds
    //RateLimiter::CheckLimitByIPExit("somemethodname", 10, 5); //Same as above, this time for a specific method
    public static function CheckLimitByIPExit($key = null, $rate_limit_cycle = 86400, $cycle_req_quota = 5000, $prolong_on_rate_limit_exceeded = false) {
        RateLimiter::CheckLimitExit((isset($_SERVER['REMOTE_ADDR']) ? $_SERVER['REMOTE_ADDR'] : null) . $key, $rate_limit_cycle, $cycle_req_quota, $prolong_on_rate_limit_exceeded);
    }
}
?>